The present invention relates to adhesive formulations for coating onto substrates of pressure-sensitive adhesive products, to pressure-sensitive adhesive products made from such formulations, and more particularly to storage-stable high solids adhesive formulations which may be used to prepare pressure-sensitive products exhibiting a balance of good peel and sheer resistance, good tack and coatable viscosity. The invention further relates to novel methods for preparing adhesive formulations and to adhesive formulations and products which perform well at high temperatures.
The invention includes pressure-sensitive adhesives based upon polyacrylate copolymers. Adhesives of this basic type are known for their ability to provide desired tack without the addition of a tackifying resin and for good clarity and lack of color. Prior art adhesives of this type are sometimes sensitive to heat and solvents, and may be adversely affected thereby.
Pressure sensitive adhesives may be sold in solution form for application to desired substrates For environmental and economic reasons, it is desirable that the solution-based acrylic adhesive be made and applied at a solids level as high as is practicable. However, a proper balance of solution viscosity and adhesive properties has proven difficult to achieve at a high solids levels. Storage stability, particularly in the presence of reactive crosslinkers, has also proven difficult at such levels.
The prior art has attempted to solve these problems in a number of ways, but without entirely satisfactory results. For example, many high solids adhesives based upon acrylic latices have rheological deficiencies and frequently lack a proper balance of peel and shear resistance, and tack.
Higher solids solvent-based acrylic pressure sensitive adhesives, e.g. 58% solids (by weight) and higher, have viscosities so high as to make application difficult. While lower viscosities may be obtained through the use of lower molecular weight polymers, it becomes difficult to obtain a desirable balance of physical properties in the final adhesive product. Shear resistance, in particular, is sacrificed.
The use of chain transfer agents during polymerization reactions involved in the production of certain adhesive formulations (or precursors thereto) is known, for example, from U.S. Pat. Nos. 4,316,830, 4,140,668, 4,128,518, 3,923,752 and 3,920,600. Use of such agents to control molecular weight is disclosed in U.S. Pat. Nos. 4,810,523, 4,702,496, 4,608,111, 4,322,472 and 4,230,772.
U.S. Pat. No. 4,305,996 relates to adhesives made from hydroxyl-containing liquid polymers. A mercaptan compound is used as a chain transfer agent during preparation of polymers.
U.S. Pat. No. 4,665,127 discloses pressure sensitive adhesive compositions which include polymers with reactive silicon-containing groups.
U.S. Pat. No. 4,042,554 relates to preparation of mastic adhesives, noting that a larger amount of chain transfer agent is needed for some purposes.
U.S. Pat. Nos. 4,005,247 and 3,769,254 relate to preparation of pressure sensitive adhesives by reacting certain acrylic interpolymers with certain metal alkoxides.
U.S. Pat. No. 3,532,708 relates to room temperature crosslinkable pressure sensitive adhesives prepared in an organic solvent and a metal alkoxide.
U.S. Pat. Nos. 3,707,518 and 3,617,362 relate to self crosslinking pressure sensitive adhesives comprising alkoxy silyl crosslinking monomers.
European Patent Publication No. 0 183 495 relates to alkyl acrylate pressure sensitive adhesive compositions containing organosilane additives.
Methyl acrylamidoglycolate methyl ether (MAGME.RTM. 100 available from Cyanamid, Wayne, N.J.; See Cyanamid's brochure: "MAGME 100 Multi-Functional Acrylic Monomer") as a co-monomer in acrylic coatings is disclosed, for example, by U.S. Pat. No. 4,454,301. According to a technical bulletin, (American Cyanamid--"Chemistry & Coatings Applications of MAGME.RTM. 100 Crosslinkable Monomer"), both the methyl ester and methyl ether groups in MAGME.RTM. monomer undergo acid catalyzed reaction with alcohols. The monomer is also believed to undergo self-condensation. It is believed that acid catalysis promotes the crosslinking reaction, and that a strong acid promotes increased crosslinking.
For purposes of pressure sensitive adhesive products, strong acid catalysts are usually to be avoided. Use of strong acid catalysts, at levels up to 3% on weight of total resin solids, carries with it the risk of impaired or shortened shelf life, and for this reason, is not well-suited for a commercially useful acrylic pressure sensitive adhesive.